Use thread context classloader when available

[acmi]

📜 Description

Prioritizes the thread context classloader over the system classloader when the bootstrap classloader is null.
This ensures correct resource loading in environments like Spring Boot where the thread context classloader is used for resource loading.

💡 Motivation and Context

Fixes #4318

💚 How did you test it?

Manually

📝 Checklist

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[lcian]

Thanks for the contribution @acmi !!